Myotonic dystrophy (DM) is the most common cause of adult-onset muscular dystrophy. This multi-systemic disorder is marked by a debilitating and progressive loss of skeletal muscle function. The major form of DM is caused by expanded CTG repeats located in the 3'UTR of the DMPK gene. It is now clear that after transcription the expanded repeats remain trapped in the nucleus and lead to a trans-dominant misregulation of alternative splicing. Although many symptoms in DM, such as myotonia and insulin resistance, are explained by particular misregulated splicing events, the mechanism by which skeletal muscle degeneration arises remains unknown. This proposal will investigate the cause of muscle degeneration using a novel transgenic mouse model generated in the sponsor's lab. In Specific Aim 1 two mouse lines will be bred to produce bi-transgenic animals which upon tamoxifen induction express 960 CUG repeats specifically in skeletal muscle. Using a number of cellular, molecular and functional assays bitransgenic mice will be tested to see how well they recapitulate the DM phenotype. In Specific Aim 2, the role of defective satellite cells in DM muscle degeneration will be examined. Satellite cells are a subpopulation of muscle cells which proliferate, differentiate and fuse with existing myotubes in order to repair injured muscle. Exercise, muscle damage and degenerative disorders are known to activate satellites cells;however in DM they fail to be activated. Satellite cells will be isolated from our bi-transgenic mice, and mechanisms leading to defective activation will be further studied. In Specific Aim 3 a splicing factor, CUGBP1, which regulates developmental alternative splicing patterns in post-natal development, will be investigated. Although CUGBP1 levels decrease in adulthood, they remain unexpectedly high in DM patients and correlate with misregulation of certain alternative splicing events. CUGBP1 steady state levels will be assayed in bi-transgenic mice after tamoxifen treatment. A potential mechanism for increased CUGBP1 protein levels via phosphorylation through the PKC alpha pathway will also be examined. The proposed work aims to elucidate the pathogenic mechanisms responsible for skeletal muscle degeneration in the leading cause of adult-onset muscular dystrophy, Myotonic Dystrophy. Information gained from this project may also provide insight into other tri-nucleotide expansion repeat disorders, such as Huntington's Chorea, Friedreich's Ataxia, Spinal Cerebellar Ataxia and Fragile X syndrome.